Debugging Cygwin Programs

When your program doesn't work right, it usually has a "bug" in
it, meaning there's something wrong with the program itself that is
causing unexpected results or crashes. Diagnosing these bugs and
fixing them is made easy by special tools called
debuggers. In the case of Cygwin, the debugger
is GDB, which stands for "GNU DeBugger". This tool lets you run your
program in a controlled environment where you can investigate the
state of your program while it is running or after it crashes.
Crashing programs sometimes create "core" files. In Cygwin these are
regular text files that cannot be used directly by GDB.

Before you can debug your program, you need to prepare your
program for debugging. What you need to do is add
-g to all the other flags you use when compiling
your sources to objects.

Example 4.4. Compiling with -g

bash$ gcc -g -O2 -c myapp.c
bash$ gcc -g myapp.c -o myapp

What this does is add extra information to the objects (they get
much bigger too) that tell the debugger about line numbers, variable
names, and other useful things. These extra symbols and debugging
information give your program enough information about the original
sources so that the debugger can make debugging much easier for
you.

To invoke GDB, simply type gdb myapp.exe at the
command prompt. It will display some text telling you about itself,
then (gdb) will appear to prompt you to enter
commands. Whenever you see this prompt, it means that gdb is waiting
for you to type in a command, like run or
help. Oh :-) type
help to get help on the commands you can type in, or
read the [GDB User's Manual] for a complete
description of GDB and how to use it.

If your program crashes and you're trying to figure out why it
crashed, the best thing to do is type run and let
your program run. After it crashes, you can type
where to find out where it crashed, or
info locals to see the values of all the local
variables. There's also a print that lets you look
at individual variables or what pointers point to.

If your program is doing something unexpected, you can use the
break command to tell gdb to stop your program when it
gets to a specific function or line number:

Example 4.5. "break" in gdb

(gdb) break my_function
(gdb) break 47

Now, when you type run your program will stop
at that "breakpoint" and you can use the other gdb commands to look at
the state of your program at that point, modify variables, and
step through your program's statements one at a
time.

Note that you may specify additional arguments to the
run command to provide command-line arguments to
your program. These two cases are the same as far as your program is
concerned: